1. Field of the Invention
The present invention relates to a spark ignition system for an internal combustion engine (hereinafter referred to as an engine).
2. Description of the Prior Art
Ignition systems are known in the art in which a current is supplied from a battery to the primary winding of an ignition coil and the contact points of a breaker connected in series with the primary winding are opened rapidly to induce a high tension voltage in the secondary winding of the ignition coil by the action of electromagnetic induction. This high voltage is applied across a spark plug to spark to ignite the mixture. With the conventional ignition systems, the primary current to the ignition coil is switched on and off by closing and opening the contact points of the breaker by a cam. The cam rotational angle in which the contact points are closed is called a dwell angle, and since this dwell angle is determined in accordance with the shape of the cam and the gap width of the contact points, the dwell angle does not depend on the engine speed and it is thus fixed. Consequently, the time during which the primary current flows into the ignition coil decreases in inverse proportion to an increase of engine speed. Since the rising characteristic of the primary current is fixed independently of the engine speed, the primary current flow decreases with the result that the primary current decreases as the engine speed gets higher and hence the secondary voltage decreases.
To overcome this difficulty, a method has been proposed in which the dwell angle is increased to increase the primary current. With the conventional ignition systems, however, if the gap width of the breaker contact points is reduced to increase the dwell angle, at the moment that the breaker contact points are opened an arc is produced between the contact points by the circuit inductance, so that the secondary voltage is decreased and hence the contact points are burnt out, thus deteriorating the ignition performance. Therefore, the method of reducing the gap width between the contact points cannot increase the dwell angle to any satisfactory extent.
Therefore, transistorized ignition systems have been proposed in which a transistor is employed to switch on and off the primary current of the ignition coil. In one form of these transistorized ignition systems, the angular position of the engine output shaft is detected by a suitable method, whereby a transistor into which the primary current is flowing is rendered nonconductive at a proper instant to switch off the primary current, and at the expiration of a predetermined time (a time T.sub.o in FIGS. 1 and 2) after the secondary voltage is generated causing a discharge at the spark plug, the transistor is again rendered conductive and the primary current again starts flowing into the transistor. In this case, since this system is not one in which the primary current is interrupted by the contact points, the dwell angle can be made considerably greater than one obtained in the conventional ignition systems.
However, as will be seen from a comparison between FIGS. 1 and 2, with transistorized ignition systems of the above type, the dwell angle is decreased at high engine speeds with respect to the dwell angle at low engine speeds. The time of flow of the primary current per unit period is longer at low engine speeds and this results in an increased quantity of heat generation in the ignition coil due to the primary current. Therefore, this conventional type of transistorized ignition system is not preferred, since the consumption of primary current and the generation of heat in the ignition coil are increased at low engine speeds.
It will thus be seen from the foregoing that an ignition system is preferred that ensures the dwell angle which is decreased at low engine speeds and which is increased at high engine speeds.